c-axis penetration depth in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals measured by ac-susceptibility and cavity perturbation technique

The c -axis penetration depth ∆ λ c in Bi 2 Sr 2 CaCu 2 O 8+ δ (BSCCO) single crystals as a function of temperature has been determined using two techniques, namely, measurements of the ac-susceptibility at a frequency of 100 kHz and the surface impedance at 9.4 GHz. Both techniques yield an almost linear function ∆ λ c ( T ) ∝ T in the temperature range T < 0 . 5 T c . Electrodynamic analysis of the impedance anisotropy has allowed us to estimate λ c (0) ≈ 50 µ m in BSCCO crystals overdoped with oxygen ( T c ≈ 84 K) and λ c (0) ≈ 150 µ m at the optimal doping level ( T c ≈ 90 K).